The present invention relates to an apparatus for optically recording information on and reproducing information from two different information disks (discs). More particularly, the invention relates to a compatible apparatus for recording and reproducing information on both a video disk and a digital audio disk.
Recent years have seen an increasing use of systems which optically record information on and/or reproduce information from a recording medium. Typical examples of such optical recording systems are video disk and digital audio disks. On the video disk, video and audio information are recorded in the form of analog signals. On the digital audio disk, on the other hand, audio information is recorded in the form of a digital signal.
In view of the similarity of the mechanisms used in driving these two types of disks and of the methods for optically recording and reproducing these signals, a demand has arisen in which a single optical recording and reproducing apparatus be provided which is usable for both the video disk and the digital audio disk. Thus, a compatible player has been provided.
However, if an apparatus is used with a video disk (or digital audio disk), operational characteristics are not satisfactory when this apparatus is used for reproducing information on the digital audio disk (or video disk) since the two disks use different formats for information recording. The information to be recorded on the video disk is an FM-modulated television signal, whereas on the digital audio disk, an EFM (Eight-to-Fourteen modulation) modulated audio disk (this may include a video or other signals), or the digital signal is recorded. Furthermore, the video disk differs from the digital audio disk not only in such design parameters as thickness, outside diameter, weight, pitch of adjacent tracks, reflectance and the diameter of the center hole but also in terms of the material of which the disks are made.
Due to the foregoing differences between the video disk and the digital audio disk, various problems are encountered in providing a compatible recording/reproducing apparatus.
Firstly, if the inclination of an optical axis of the laser beam with respect to the disk is adjusted optimumly for either one of the video and digital audio disks, no optimum information reproduction is obtained for the other disk, with the result that information cannot be reproduced on one of the disks with good S/N ratios. This problem is caused by the fact that the video disk and the digital audio disk are different in outer diameter whereby the video disk being loaded sags by a different degree than the digital audio disk.
More specifically, in an optical recording/reproducing apparatus, an angle between the optical axis and the disk must be maintained in a certain relation, and ideally they are perpendicular to each other. FIGS. 1A and 1B illustrate an arrangement for realizing such angular adjustment on a digital audio disk player. In the Figures, reference numeral 1 denotes a digital audio disk as an information recording disk, and 2, a pickup as record/reproduce means. The pickup 2 has an objective lens 3 from which a laser beam 4 is directed against the disk 1 so as to reproduce information. Numeral 5 denotes a light source such as a light-emitting diode (FIG. 1B) and light 6 emanating from the light source is reflected from the disk 1 and received by two light-receiving elements 7 and 8. These elements constitute an angle detection means 9 for detecting the relative angle between the optical axis (pickup 2) of the laser beam and the disk 1.
The angle detection means 9 is so designed that if the optical axis has a certain angle with respect to the disk 1 (ideally, they are perpendicular with respect to each other) and when desired characteristics (e.g. jitter characteristics) are obtained, the difference between the output signals from the light-receiving elements 7 and 8 that are produced as a result of incidence of light reflected from the disk 1 after issuance from the light source 5 is zero. Therefore, if the relative angle of the optical axis fails to satisfy the predetermined relation with the disk, the balance between the amounts of light received by the elements 7 and 8 changes and a corresponding differential signal appears. By operating drive means (not shown) for the pickup 2 in response to this signal, servo control can be realized so that the optical axis is maintained to have the predetermined angle with respect to the disk.
In the mode of reproduction on a video disk, it is preferred to reproduce an RF (HF) signal having the highest possible level. On the other hand, with the digital audio disk, the amount of jitter should be minimized. The relative angle .theta.v between the optical axis and the disk that ensures the reproduction of the highest level of RF signal on the video disk is not necessarily the same as the angle .theta.a that reduces the amount of jitter to minimum level during reproduction on the digital audio disk. Therefore, if the reference position for the angle of the optical axis (pickup 2) is set to a value that is optimum for the video disk, the amount of jitter that occurs during reproduction on the digital audio disk cannot be reduced to a minimum level. On the other hand, if the reference position is set to a value that is optimum for the digital audio disk, no RF signal having a maximum level can be reproduced on the video disk (see FIG. 2).
Secondly, when the pickup 2 moves on the disk 1 n its radial direction and reaches a point close to the outermost periphery as shown in FIG. 1B, the light-receiving element 8 in the angle detection means 9 comes to be positioned outside the disk even if the objective lens 3 is positioned within the boundary of the disk 1 and this causes failure in the reception of the light from the light source 5 to element 8, which may lead to misoperation of the recording apparatus. The potentional of such misoperation is increased as the diameter of the disk decreases, so in designing a "compatible" recording/reproducing apparatus all parameters must be determined on the basis of the smaller-diameter disk, i.e., digital audio disk. This, however, restricts the position where the angle detection means is installed, thereby reducing the freedom in designing the apparatus.
Thirdly, due to the fact that the track pitch of adjacent recording tracks varies with the type of disk, the following problems are encountered.
The optical information recording/reproducing system uses the pickup as optical means, and in order for this pickup to accurately follow the recording tracks on the information recording disk, a tracking servo system generating a tracking error signal is provided, and on the basis of this error signal, the position of the pickup in the radial direction of the disk with respect to the recording tracks is controlled. The "three-beam" system is currently used to generate the tracking error signal.
FIGS. 3 and 4 are illustrations of the principle and operation of the three-beam system. In FIG. 3, reference numeral 11 indicates an information pit forming part of a recording track. In addition to a recording information detecting beam spot 12, two tracking information detecting beam spots 13 and 14, one preceding the spot 12 and the other following spot 12, are applied from the pickup (not shown) to illuminate the recording surface of the disk.
In FIG. 4, the light spot 12 reflected from the recording surface is applied to a "four-division" type photodetector 15, and four outputs from the photodetector 15 are summed in an adder (not shown) to provide a read-out signal. Photodetectors 16 and 17 that respectively detect the light intensities of the beam spots 13 and 14 produce outputs A and B, which are fed into a subtractor 18 where the difference between A and B is taken to produce a subtraction output C which is subsequently used as a tracking error signal.
As shown in FIG. 5, if the track pitch of recording tracks 19 formed by a series of information pits 11 is written as "a" and when the distance between the beam spot 12 and each of spots 13 and 14 in the radial direction of the disk is written as "b", then, by setting the value of distance "b" as a fraction, e.g., one-fourth, of track pitch "a" (i.e., b=a/4), a tracking error signal is obtained, which, as shown in FIG. 6, varies with the amount by which the recording information detecting beam spot 12 is biased from the center of a recording track. In the tracking servo system, the position of the pickup relative to the recording track 19 in the radial direction of the disk is so controlled that the tracking error signal described above is reduced to zero.
Now assume the use of a pickup wherein the distance "b" between the beam spot 12 and each of the spots 13 and 14 in the radial direction of the disk is preset on the basis of the track pitch "a". If the disk used has a track pitch "a1" wider than "a", as shown in FIG. 7, the tracking error signal obtained has a gentle slope in the region about the zero-crossing point as shown in FIG. 8. If, on the other hand, the disk has a track pitch "a2" narrower than "a", as shown in FIG. 9, the tracking error signal obtained has a small absolute value as shown in FIG. 10. Special modes of operation such as track jump and the control of tracking servo gains have to be performed periodically on the basis of tracking error signals, but if the tracking error signals undergo variations of the types described above, there is a high likelihood that the desired special modes of operation cannot be realized. In this connection, it should be noted that the relative positions of the beam spots 12, 13 and 14 cannot be easily altered.
Lastly, there exists a problem in the compatible optical recording/reproducing apparatus due to a difference in the optical system adapted to reproduce information from the video disk and the digital audio disk.
Specifically, a numerical aperture NA of the objective lens is generally determined by the space frequency of the video disk the reproduction mode. On the other hand, the amount of jitter is the predominant factor in the case of the digital audio disk. Therefore, in designing a compatible player capable of operating both the video disk and the digital audio disk in the reproduction mode with a single optical system, if the numerical aperture NA of the objective lens is set to a value optimum for one disk, the value is by no means optimum for the other disk and a signal with good S/N ratio cannot be obtained from the latter disk.
Two methods have conventionally been employed to deal with this problem: one is to use different types of objective lens 24 for reproducing information from different disks; and the other method is to employ a retractable corrective lens 28 between the beam splitter 23 and the objective lens 24 as shown in FIG. 11. In this second approach, the corrective lens 28 is retracted from the present optical path during reproduction on the video disk so as to provide an optical path A having an increased numerical aperture whereas in the mode of reproduction on the digital audio disk, the lens 28 is inserted into the present optical path so as to provide an optical path B having a decreased numerical aperture. However, the arrangement shown in FIG. 11 requires a complicated optical system which leads to a larger, and more costly apparatus.
In FIG. 11, reference numeral 21 denotes a light source from which a laser beam emanates and is passed through a collimator lens 22, a beam splitter 23, the corrective lens 28 (if projected), and the objective lens 24 to be focused on the disk 25. The laser beam reflected from the disk 25 travels back through the objective lens 24 and the corrective lens 28 (if projected) to enter into the beam splitter 23, where it is reflected and retracted to go to a condenser lens 26, from which the beam is focused on a light receiving element 27.